Peripheral nerve blockade provides anesthesia of a region of the body by applying local anesthetic to a selected nerve or plexus of nerves. Growing popularity of this technique is due to its safety relative to general anesthesia, suitability for outpatient surgical care, and the prolonged postoperative pain relief it provides.
The main technical challenge for safe and effective peripheral nerve blockade is the placement of a needle close to the target nerve without entering the nerve, for the purpose of injecting a volume of anesthetic solution. Identification of the proper site for injection involves two sequential phases. First, the place for needle puncture of the skin must be determined. Secondly, the needle must be advanced in the proper direction and to the proper depth prior to injection. The skin puncture site is typically determined by identifying relevant landmarks using visual examination and palpation through the skin. These landmarks include contours of muscles and bones, and arterial pulses. The second phase of needle insertion uses a different repertoire of techniques for perfecting the placement of the needle. Contact with bones or vessels is used when these landmarks are known to be close to the target nerve. Alternatively, needle contact with the nerve may be sought, in which case the provoked sensation confirms needle placement. Finally, current may be passed through the needle to stimulate the nerve electrically. This creates a radiating sensation and contraction of the muscles innervated by the stimulated nerve. In most cases, this final technique, electrical nerve stimulation, is used for final needle placement.
While the techniques above are widely used, success is highly variable. For instance, a 20% failure rate is typical for brachial plexus blockade even after substantial experience is acquired (Konrad 1998). Because nerve stimulation through the needle is a highly reliable method of needle identification, the current limitation is the first phase of the process, namely choosing the correct initial needle puncture site. Human anatomy is highly variable, such that nerves and the structural landmarks used to find them are not consistently placed. Additional challenges are faced when the subject is large or if injury has distorted structure. Since the needle, once inserted, can be used to search a zone only about a 1 cm wide, many failures are do to the initial placement of the needle puncture through the skin at a site too distant from the nerve. In addition to pain and discomfort, damage to various organs may ensue.
The remedy for the limitations discussed above is improved identification of the optimal needle insertion site. If this can be specified within 1 cm or less, the well-established methods for directing needle manipulation, particularly electrical stimulation through the needle, can be used with high success.
Stimulation for the purpose of identifying the location of a nerve for blockade has been proposed in published papers. In one case [Bosenberg et al, Paedriatric Anaesthesia 2002, 12:398–403], application of current to the skin to map the nerve is done prior to cleaning the skin for puncture and is not compatible with simultaneous palpation or compression with the nondominant hand. In another publication [Urmey et al, Regional Anesthesia and Pain Medicine 2002, 27:261–7], a device is described that stimulates transcutaneously and through which the injection needle is passed. This method prevents the use of the nondominant hand for any purpose other than holding the stimulating device against the skin by its handle.